The invention relates to a process for curing sand moldings, in particular casting cores, made from synthetic resin-bound sand by means of gas or vapor form catalyst which is added to a carrier gas, after which the catalyst/carrier gas mixture is forced through the mold containing the loose sand molding, the gas mixture escaping from the mold being collected as completely as possible without mixing with atmospheric air, and the catalyst component of the gas mixture being removed from the latter as completely as possible by condensation and, where appropriate, being re-used after removal of contaminants.
The process outlined in the introduction is described in detail in German Pat. No. 2,550,588. Using this so-called cold box process, cores having good flexural strength and abrasion resistance, high dimensional accuracy and surface quality and a long shelf life can be produced from cold molds in very short cycle times. The sand mixture for this process comprises dry quartz sand and a liquid two-component synthetic resin binder system which cures on introduction of the catalyst.
The catalysts used are highly volatile amines which are liquid at room temperature, most frequently triethylamine, dimethylethylamine or dimethylisopropylamine.
Since the amines are evil smelling and toxic, they must not be allowed to reach the environment. In addition to removal of the amines from the offgas stream of a core producing plant by acid washing, thermal afterburning or cumbustion in a cupola furnace after introduction into the hot blast stream, with the amines being lost has been contemplated. It is known from German Pat. No. 2,550,588 to remove the catalyst and other condensable vapors from the catalyst containing offgas by condensation. The catalyst can be re-used, if necessary after removal of contaminants, for example by fractional distillation.
A general disadvantage of this process is that the catalyst is only present in a relatively low concentration in the carrier gas mixture. In order to remove the catalyst, considerable cooling of the entire gas stream is therefore necessary, which is associated with high costs. The process has therefore not been able to establish itself in industry, even in spite of the high price of the catalyst.
An object of the invention is therefore to make it possible to recover the catalyst more inexpensively in a process for curing sand moldings. This object is achieved in a process wherein the gas mixture escaping from the mold is passed over the mixture feed side of a semipermeable membrane while a pressure is maintained on the permeate side of the membrane which is lower than the pressure on the mixture feed side, whereby the catalyst vapors preferentially permeate through the membrane, and a permeate carrier gas stream which is greater than the permeate stream passing through the membrane is passed over the permeate side of the membrane, and wherein the catalyst vapors are recovered from the permeate carrier gas stream.
An important aspect of the invention is thus that the catalyst/carrier gas mixture escaping from the mold is no longer condensed directly, but instead that, after permeation through a semipermeable membrane, a catalyst concentrated gas mixture is produced from which the catalyst can then be recovered at lower cost, for example by condensation. For this purpose, the gas mixture escaping from the mold is passed over the mixture feed side of a semipermeable membrane which preferentially allows the catalyst components of the gas mixture to pass through (permeate). A membrane of this type which is suitable in the present process comprises, for example, polydimethylsiloxane.
The driving force for permeation of the catalyst is formed by a partial pressure gradient between the mixture feed side of the membrane and the reverse side, the so-called permeate side. This concentration gradient is established by maintaining a lower pressure on the permeate side of the membrane than prevails on the mixture feed side. In addition, the partial pressure gradient on the permeate side is further increased by passing over the permeate side of the membrane a permeate carrier gas stream which is greater than the permeate stream permeating through the membrane. A permeate carrier gas stream which is about 50 to 200 (liter) greater than the permeate volume passing through the membrane per m.sup.2 (meter.sup.2) and per hour is preferred. The carrier gas used is air or nitrogen. Nitrogen is therefore preferred since explosive mixtures cannot arise together with the catalyst vapors permeating. On the permeate side, a gas mixture which is highly concentrated with respect to the catalyst content is present, from which the condensable components can be removed in a manner known per se by cooling. A purification step, for example fractional distillation, may be necessary before re-use of the catalyst. The separation of substance mixtures using membranes is known in principle, for example from U.S. Pat. No. 4,553,983, in which the recovery of solvents, in particular from paint shops, is described. Surprisingly, the principles described therein on permeation can also be applied according to the invention to permeation of the highly polar, basic amines used as catalyst in mold production, although paint solvents are neutral and essentially non-polar.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.